1. Field of the Invention
The present invention generally relates to a control apparatus for a throttle valve of an automobile engine. More specifically, the invention is directed to an apparatus for controlling a pair of throttle valves, one of which is electronically controlled by an electrically controlled actuator and the other of which is mechanically controlled by an accelerator pedal.
2. Description of the Related Art
In recent years, as means for controlling the engine to improve the exhaust emission controlling performance and fuel consumption performance, there has been developed a throttle valve controlling apparatus using an electrically controlled actuator in which a throttle valve and an accelerator pedal are not mechanically coupled, but the opening and closing of the throttle valve are controlled based upon a control signal. This control signal is obtained by converting an accelerator pedal operating amount into an electric signal (this signal is referred to as "an accelerator pedal operating amount signal") and signals indicative of the other engine operating state or vehicle running state, for instance, these signals include an engine revolution speed signal, a gear position signal, and the like.
The throttle valve is opened and closed by a throttle valve drive motor. This motor operates in accordance with a command which is input from a vehicle controlling apparatus consisting of a calculation/controlling unit to sequentially calculate the optimal opening degree in correspondence to the engine operating state or vehicle running state. Therefore, it is necessary to provide a fail-safe device to prevent the runaway of the vehicle even if the electrically controlled actuator including the drive motor is made inoperative during the running of the vehicle. As a conventional example having such a fail-safe device, there has been disclosed a specific construction in Japanese Patent Publication No. 58-25853 (1983). That is, the electromagnetic clutch is employed to disconnect the throttle valve shaft from the drive motor when the engine becomes uncontrollable, and the return spring to fully close the throttle valve when the electromagnetic clutch is in the inoperative mode, is attached to the throttle valve shaft. On the other hand, in Japanese Patent Disclosure (KOKAI) No. 61-215436, there has also been disclosed the apparatus in which the throttle valve shaft is mechanically rotated through a differential gear device in response to a control amount of the electrically controlled actuator or an operating amount of the accelerating pedal.
However, according to such conventional apparatuses, when the electrically controlled actuator is once made inoperative, there is another problem such that the runaway of the vehicle cannot be avoided even if the electrically controlled actuator is normally operable, but the calculation/controlling unit for controlling this actuator is brought into the uncontrollable condition.
When the throttle valve is returned to the fully closed position due to a failure or abnormality of an electrically controlled actuator and/or calculation/controlling unit provided in a conventional throttle valve controlling apparatus, although the runaway of the vehicle can be prevented, there is a still problem such that the continuous driving of the vehicle cannot be maintained and thus the vehicle cannot be moved to a repair shop.
The present invention is made to solve the foregoing problems and it is an object of the invention to obtain a throttle valve controlling apparatus of an engine in which even if an electrically controlled actuator and/or calculation/controlling unit is brought into malfunction, the runaway of the vehicle can be prevented, and even during such a malfunction the vehicle can be continuously driven (i.e., so-termed "limp home" driving). That is, an object of the invention is to obtain a throttle valve controlling apparatus which can execute such a limp home drive of the vehicle.